In the field of magnetic recording, realization of digital recording little in the deterioration of records is progressing superseding conventional analog recording. In addition to higher image quality and sound quality, miniaturization and saving of space are also required of the recording and reproducing apparatus used for digital recording and the magnetic recording medium. In general, much signal recording is required in digital recording as compared with analog recording, so that further higher recording density is required of the magnetic recording medium for digital recording.
An MR head that works with MR (magneto-resistance) as the principle of operation have got to be used in recent years. Several times of reproduction output can be obtained with the MR head as compared with the induction type magnetic head. Further, noises generated from instruments, e.g., impedance noises, are greatly reduced, therefore, it becomes possible to realize good recording and reproduction by lowering the noises coming from magnetic recording media themselves, and high density recording characteristics can be drastically improved. For this purpose, further fining of ferromagnetic powder particles and smoothing of the magnetic layer surface are required of the magnetic recording media for suppressing the noises of the media themselves.
In addition, for achieving high density recording, further shortening of the wavelengths of recording signals and narrowing of the recording track are necessary, so that further fining of ferromagnetic powder particles, increasing of packing density and smoothing of the magnetic layer surface are also required from these points of view. Further, in a magnetic tape, thinning of a magnetic recording medium is also required for increasing the volume density.
A magnetic recording medium has generally a constitution comprising a nonmagnetic support having provided thereon a magnetic layer, or a nonmagnetic support having thereon a nonmagnetic layer and a magnetic layer on the nonmagnetic layer.
For the thinning of a magnetic recording medium with the increase of recording density, not only a magnetic layer but also a nonmagnetic support and a nonmagnetic layer are required to be thin. However, mere thinning is accompanied by the reduction of running durability due to thinning of a nonmagnetic support, the surface state of the nonmagnetic support greatly influences the magnetic layer due to thinning of the magnetic layer and the nonmagnetic layer, and the surface smoothness of the magnetic layer is disturbed, e.g., spines occur by reflecting the surface state of the nonmagnetic support, which results in the reduction of output, and the generation of dropout and noise. In particular, in a magnetic tape for use in a linear recording system, a tape runs almost in parallel to a head to touch the head, so that the dropout is liable to occur due to the spines on the surface of the magnetic layer.
On the other hand, for obtaining the smoothness of the surface of a magnetic layer, it is an effective means to smooth the surface of the side of the nonmagnetic support on which the magnetic layer is formed, and further fining of additives, such as fillers, to be added to the nonmagnetic support and further reduction of the addition amounts are effective for that purpose. However, the strength of the nonmagnetic support lowers by changing the filler size and addition amount, and durability, particularly the durability of the tape edge, is deteriorated.
Accordingly, in advancing the increment of recording density of a magnetic recording medium, the compatibility of the improvement of electromagnetic characteristics by surface smoothness and durability is desired.
A magnetic recording medium is so far proposed (refer to, e.g., JP-A-7-6351 (The term “JP-A” as used herein refers to an “unexamined published Japanese patent application”.) and JP-A-8-45060) in which the strength of the nonmagnetic support is ensured by justifying the molecular weight and the viscosity of the polyethylene naphthalate used as the nonmagnetic support and the Young's modulus of the nonmagnetic support, and particularly the assurance of running durability is contrived by preventing the protuberance of the tape edge.